GRAINY footage of police officers shooting members of the public, often black, has become unhappily familiar in recent years. Smartphones, which have proliferated, enable anyone to record police actions. The footage of the death of Keith Lamont Scott, which prompted violent protests in North Carolina last month, is striking for another reason—it comes from the police. It is increasingly common for police officers to sport a camera on their uniforms. A growing body of evidence suggests that the gadgets improve the behaviour both of cops and those they deal with.

A study published by researchers at the University of Cambridge at the end of September suggests that body cameras can slash the number of complaints made about the police. Over the course of the year of their study some 2,000 officers in two forces in America and four in Britain were randomly assigned cameras according to their shift. Compared with the previous year, the number of complaints brought against them dropped by 93%. They also fell when officers were not wearing them during the trial, an effect the authors call “contagious accountability”. To ensure the cameras’ success, cops should have little discretion as to when they turn them on and off. According to Barak Ariel, one of the researchers on the Cambridge study, officers who wore cameras but only started recording in the middle of their interactions with the public were more likely to use force than those not using them.

Civil-liberty campaigners welcome the chance to keep an eye on the police. Many police forces are enthusiastic too. Dealing with complaints is expensive. Cameras improve the behaviour of members of the public and reduce the number of bogus complaints brought against the police. They are an efficient way to collect evidence. They can be used in training; officers can learn from their colleagues’ actions. Mr Ariel reckons British cops are more open to the devices than their American counterparts. Police unions in Boston and Cincinnati say they should not be rolled out until their contracts are changed to reflect the new work that cameras will demand.

Yet the use of cameras brings new challenges. If police record every interaction with the public, they will have to find a way to store the many hours of footage generated. Questions will then arise as to how long such data should be kept and in what circumstances they should be released to the public. Such worries are not insignificant. But the evidence is mounting that the usefulness of such cameras outweighs the concerns.